Seismic surveys image or map the subsurface of the earth by imparting acoustic energy into the ground and recording the reflected energy or “echoes” that return from the rock layers below. The source of the acoustic energy can be generated by explosions, air guns vibrators, and the like. The energy source is positioned on or near the surface of the earth. Each time the energy source is activated it generates a seismic signal that travels into the earth, is partially reflected, and, upon its return, may be detected at many locations on the surface as a function of travel time. The sensors used to detect the returning seismic energy commonly include geophones, accelerometers, and hydrophones. The returning seismic energy is recorded as a continuous signal representing displacement, velocity, acceleration, or other recorded variation as a function of time. Multiple combinations of energy source and sensor can be subsequently combined to create a near continuous image of the subsurface that lies beneath the survey area. One or more sets of seismic signals may be assembled in the final seismic survey.
The acquisition of seismic data is an expensive undertaking and any time savings can amount to significant cost savings. One significant time saver has been the development of phase separated seismic prospecting which makes it possible to acquire seismic data from a number of shot points simultaneously to increase the number of shot points shaken in a day and ultimately substantially reduces the number of days to acquire the data for a survey area. While the amount of time that the vibes spend at a single shot point is longer, at the end of the series of sweeps, several shot points have been “completed” and the vibes move on to another setup of shot points to shake.
Each excitation of the vibrator is known as a “sweep” (or sometimes called a “chirp sweep” if really short). Although many sweep patterns are possible, a common one is the “linear” sweep, which is designed to vary between two frequency limits (e.g., between 5 Hz and 150 Hz) over a predetermined period of time in a linear or smoothly varying manner. Other sweeps apply different biases to the duration of time spent in individual frequencies such that more or less time is spent on those frequencies. These are commonly referred to as non-linear sweeps or “gain sweeps”. The amplitude of the sweep signal might either be fixed or frequency dependent, depending on a number of factors well known to those of ordinary skill in the art. For example, a sweep is characterized by a starting frequency and ending frequency, tapers and a sweep duration. The moment a sweep starts is also helpful information to separate sweeps.
Technology continues to increase resolution and complexity of seismic systems such as high fidelity vibroseis seismic acquisition including ZENSEIS®. Vibroseis is a method used to propagate energy signals into the earth over an extended period of time as opposed to the near instantaneous energy provided by impulsive sources. The data recorded through vibroseis method must be correlated to convert the extended source signal into an impulse. The source signal using this method was originally generated by a servo-controlled hydraulic vibrator or similar shaker unit mounted on a mobile base unit, but electro-mechanical and pure electric versions have also been developed. Signals transmitted through the earth are reflected and analyzed to identify changes in signal. The exact distance the vibrations travel before being reflected are unknown and the transmission rates of the vibrations through different features is unknown, thus the time from transmission of the signal to recording of the seismic signal is the only direct measure of distance. The exact time is additionally required to extract phase data when more than one vibroseis or other vibrational source is operated simultaneously.
Global Positioning Systems (GPS) are currently used by military and civilians to accurately determine location, direction and rate of movement, as well as time. GPS systems have been used by seismic operators to accurately place vibratory source and sensors during seismic surveys and to provide an accurate time for a GPS survey as a single source of time. Other methods are then used to synchronize time between a central recorder, source, and receivers. These methods include high-precision microsecond time recorders, accurate radio-pulse transponders and receivers, as well as other methods of high accuracy time synchronization. Radio-pulse synchronization requires radio communication with a large number of source and autonomous sensors, requires a powered receiver at each sensor, and a very accurate clock or GPS based timing device to obtain microsecond precision among all of the integrated devices required for seismic surveying.
Prior studies have used the HFVS concept combining the unique sweep encoding advantages of ZENSEIS® and timing synchronization to coordinate source and data recorders for a higher quality survey. By combining the timing accuracy of a GPS with an inexpensive timer or clock, the size of the autonomous data recorder can be dramatically reduced and less energy is required to maintain the system. Because the system has both an accurate near microsecond timing system and inexpensive timer, the system has sufficient accuracy for an HFVS, ZENSEIS®, slip-sweep or similar high production seismic survey method, yet can overcome gaps in communication and radio signal without compromising the data recorded therein. Therefore, an autonomous, continuous recorder may be employed in a seismic survey to potentially eliminate the need of a recording truck and observer as an integral piece of hardware required on the crew. Autonomous recorders are becoming the industry standard for seismic acquisition and continuously recording units are now readily available from many vendors.
In the process of acquiring conventional seismic data, a crew is typically deployed across several tens of square miles of a survey area positioning cables and seismic receivers while seismic sources move from predetermined point to predetermined point to deliver vibrational seismic energy into the earth. The receivers capture the reflected signals that are recorded and subsequently processed to develop images of geologic structures under the surface. The use of autonomous nodes is similar in the conventional seismic method, but the nodes tend to eliminate the positioning of cables and hardware due to the independent nature of the nodes. This is their prime advantage in the field, the reduction of cables and the downtime related to keeping the cables working and undamaged.
Multiple source vibrator technology has been used in land-based seismic surveys for years, and its advantages are well documented. An example of conventional multiple source technology would be “slip-sweep” or HFVS recording. Conventional land-based seismic assays employ multiple, simultaneously energized seismic sources (e.g., trucks with vibrating baseplates) to impart vibratory energy into the ground normally controlled from a central location called the recorder. The imparted vibratory energy travels through the ground, is reflected and/or refracted by various discontinuities in the ground, and the reflected vibratory energy is detected by multiple seismic receivers (e.g., geophones) that are located on the ground at a distance from the seismic sources. The reflected vibratory energy recorded by the receivers is a composite reading representing the reflected energy originating from all the seismic sources. An important step in conventional HFVS multiple source vibrator technology is “source separation” of the composite data into discrete source specific data (i.e.: a “shot record”).
In order to allow for HFVS source separation, conventional multiple source vibrator technology requires multiple sweeps to be performed while the seismic sources maintain a fixed location. According to conventional HFVS multiple source vibrator technology, the number of sweeps performed at a fixed source location must be equal to or greater than the number of sources sought to be separated. For example, if four seismic sources are being used in a conventional multiple source vibrator acquisition scheme, at least four sweeps must be carried out for each HFVS source set-up. In addition, conventional HFVS multiple source vibrator technology normally uses orthogonal requires uniquely encoded (e.g., phase, frequency, and/or amplitude encoded) vibratory energy for each sweep, so that source separation of the resulting composite data can be performed.
Optimized phase encoded seismic sweeps by sweeping vibrators on separate source points, sometimes described as Zenseis® seismic prospecting, increases seismic survey productivity and quality over conventional seismic by acquiring data at several source points at the same time and uses optimal phase separation for an improved seismic dataset. If two phase encoded surveys are being conducted at the same time in close proximity, as long as the start times for each sweep are reasonably separated and the actual phase encoding of each crew is optimally tuned, each may generally proceed without time sharing. However, a conventional vibroseis seismic survey crew receives the phase encoded sweeps as significant noise and, heretofore, has not been able to proceed when another crew is in the area.
However, the conventional methods still require the vibratory source and the receiver/recording units to be coupled and normally co-located for synchronizing purposes. Recording trucks with observers are still needed in this process, mainly to ensure the proper recording of data and to properly initiate the sweeps in different vibratory sources (“vibes”). Therefore, there is still the need for a method of autonomous and continuous seismic survey where the receiver/recording units are decoupled from the vibratory source and eliminate the need of observers to save the operation cost.